Ultraviolet (“UV”) and near-infrared (“NIR”) absorbing alkali containing silicate glass-ceramics are a class of glass-ceramics which exhibit optical properties dependent on the wavelength of light which is incident on the glass-ceramics. Conventional UV/IR-blocking glasses (with low or high visible transmittance) are formed by introducing certain cationic species (e.g., Fe2+ to absorb NIR wavelengths and Fe23+ to absorb UV wavelengths, and other dopants such as Co, Ni, and Se to modify the visible transmittance) which are bonded with the glass network. Traditionally, these glass-ceramics were produced by melting the constituents together to form a glass, followed by the in situ formation of submicron precipitates through a post-formation heat treatment to form the glass-ceramic. These submicroscopic precipitates (e.g., tungstate- and molybdate-containing crystals) are absorptive of wavelength bands of light giving the glass-ceramic its optical properties. Such conventional glass-ceramics could be produced in both transparent as well as opalized forms.
Conventional tungsten and molybdenum alkali containing silicate glasses were believed to be bound to a specific and narrow composition range in order to produce glasses and glass-ceramics that are transparent at visible wavelengths. The believed composition range was based on a perceived solubility limit of tungsten oxide within peralkaline glass. For example, when batched and melted in a conventional manner, tungsten oxide can react with alkali metal oxides in the batch to form a dense alkali tungstate liquid at a low temperature during the initial stages of the melt immediately after being put into a melting furnace (e.g., the reaction occurs at about 500° C.). Because of the high density of this phase, it rapidly segregates at the bottom of the crucible. At significantly higher temperatures (e.g., above about 1000° C.), silicate constituents start to melt, and because of the silicate constituents' lower density, it remains atop the alkali tungstate liquid. The difference in densities of the constituents results in a stratification of the different liquids which gives the appearance to those skilled in the art of an immiscibility with one another This effect was observed particularly when R2O (e.g., Li2O, Na2O, K2O, Rb2O, Cs2O) minus Al2O3 was about 0 mol % or greater. The resulting apparent liquid immiscibility at melting temperature resulted in a tungsten-rich phase segregating and crystallizing as it cooled which manifested itself as an opalized, non-transparent, crystal. This issue was also present with molybdenum containing melts.
Those having ordinary skill in the art observed the tungsten- and/or molybdenum-rich phase separate from the silicate rich phase, they perceived a solubility limit of tungsten and/or molybdenum (e.g., about 2.5 mol %) within the silicate rich phase. The perceived solubility limit prevented the glass from ever becoming super-saturated with tungsten or molybdenum oxides, thereby preventing either constituent from being controllably precipitated through post-forming heat-treatment to produce a glass-ceramic with a crystalline phase including these elements. Thus, the perceived solubility prevented the development of glass-ceramic compositions which achieved a sufficient quantity of solubilized tungsten and/or molybdenum to allow the formation of tungsten and/or molybdenum containing wavelength dependent submicroscopic crystals through subsequent heat treatment.
In view of these limitations, there is a need for new compositions and methods of making them that facilitate improved near infrared and ultraviolet blocking (e.g., through higher tungsten and molybdenum solubility).